There exists a great variety of methods and systems used in the industry for measuring characteristics of materials, e.g., surface resistance, thickness of coating films and layers applied or laid onto substrates, etc. These methods and systems can be classified in accordance with different criteria and are described in our aforementioned earlier U.S. patent applications Ser. No. 954,550 (2001), Ser. No. 359,378 (2003); and Ser. No. 434,625 (2003).
A specific group of methods and devices for precision measurement of supper thin films, especially those used in the manufacture of semiconductor devices, is the group based on the use of inductive sensors, in particular those based on so-called resonance technology, in which parameters, e.g., of thin films are measured indirectly through variations in the resonance characteristics of the measured films in comparison to the known characteristics of the same films with known properties.
U.S. patent application Ser. No. 954,550 filed by Boris Kesil, et al. on Sep. 17, 2001 describes a system and method for measuring thickness and thickness fluctuations in conductive coatings with sensitivity as high as several hundred Angstroms. The system consists of an inductive sensor and a proximity sensor, which are rigidly interconnected though a piezo-actuator used for displacements of the inductive sensor with respect to the surface of the object being measured. Based on the results of the operation of the proximity sensor, the inductive sensor is maintained at a constant distance from the controlled surface. Variations in the thickness of the coating film and in the distance between the inductive sensor and the coating film change the current in the inductive coil of the sensor. The inductive sensor is calibrated so that, for a predetermined object with a predetermined metal coating and thickness of the coating, variations in the amplitude of the inductive sensor current reflect fluctuations in the thickness of the coating. The distinguishing feature of the invention resides in the actuating mechanism of microdisplacements and in the measurement and control units that realize interconnection between the proximity sensor and the inductive sensor via the actuating mechanism. The actuating mechanism is a piezo actuator. Measurement of the film thickness in the submicron range becomes possible due to highly accurate dynamic stabilization of the aforementioned distance between the inductive sensor and the object. According to one embodiment, the distance is controlled optically with the use of a miniature interferometer or a fiber-optic proximity sensor, which is rigidly connected to the inductive sensor. According to another embodiment, the distance is controlled with the use of a capacitance sensor, which is also rigidly connected to the inductive sensor. To achieve a certain level of accuracy during environment temperature variations, it is recommended to provide the proximity sensor with a thermocouple for temperature control.
However, the sensor disclosed in the aforementioned patent application could not completely solve the problems associated with accurate measurement of super-thin films, e.g., of those thinner than 500 Angstroms. This is because the construction of the aforementioned sensor is limited with regard to the range of operation frequencies, i.e., the sensor cannot be used in frequencies exceeding several tens MHz. Furthermore, the system which in this apparatus is used for stabilization of the distance between the sensor and the film is rather complicated, which makes the entire system complex and expensive. But what is most important, the aforementioned complexity delays the system response in each measurement point, so that the system have low measurement efficiency and may not be suitable for used under mass production conditions.
The above problems restrict practical application of the method and apparatus of U.S. patent application Ser. No. 09/954,550 for measuring thickness of very thin films and deviations from the thickness in the aforementioned films. Furthermore, it is obvious that the aforementioned method and apparatus do not allow thickness measurement of non-conductive films. The sensor has relatively large overall dimensions and in many cases comprises a stationary measurement instrument.
In an attempt to solve the problems of the device and method disclosed in U.S. patent application Ser. No. 954,550, one of the inventors of the aforementioned Patent Application with participation of two new inventors has improved the accuracy of the method and apparatus for measuring thickness of thin films. These improved method and apparatus are disclosed in U.S. patent application No. 359,378 filed by Boris Kesil, et al. on Feb. 7, 2003. The new apparatus consists of an inductive coil having specific parameters, an external AC generator operating on frequencies, e.g., from 50 MHz to 2.5 GHz, preferably from 100 MHz to 200 MHz, and a measuring instrument, such as an oscilloscope, voltmeter, etc., for measuring output of the sensor. The coil has miniature dimensions. The invention is based on the principle that inductive coil of the sensor, active resistance of the coil winding, capacitance of the inductive coil (or a separate capacitor built into the sensor's circuit), and the aforementioned AC generator form a parallel or a series oscillating circuit. The main distinction of the sensor of the last-mentioned invention from all conventional devices of this type is that it operates on very high resonance frequencies or on frequencies close to resonance frequency, preferably within the range of 100 to 200 MHz. In order to maintain the aforementioned high frequency range, the oscillating circuit should have specific values of inductance L (several nano-Henries) and capacitance C (several pico-Farades), and in order to provide accurate measurements, the Q-factor on the above frequencies should exceed 10. It has also been found that on such frequencies the capacitive coupling between the coil of the oscillating circuit and the virtual coil induced in the films acquires the same weight as the mutual inductance between the both coils. In other words, the system can be described in terms of inductive-capacitive interaction between the sensor and the film to be measured. The capacitive-coupling component determines new relationships between the parameters of the film, mainly the film thickness, and parameters of the resonance oscillating circuit. By measuring the parameters of the resonance oscillating circuit, it becomes possible to measure film thickness below 500 Angstroms, as well as other characteristics of the film.
However, in the apparatus of U.S. patent application Ser. No. 359,378 the methods and system for stabilization of the distance between the sensor and the surface of the film being measured remains the same as in the system of first-mentioned U.S. patent application Ser. No. 954,550, and this feature limits significant potentials of the new method and system.
The method and apparatus aimed at still further improvement of properties disclosed in aforementioned U.S. patent application Ser. No. 359,378 are described in new U.S. patent application Ser. No. 386,648 filed by the same applicants as in the previous application on Mar. 13, 2003. This new apparatus allows highly accurate and efficient contactless measurement of film thicknesses below 1000 Angstroms by means of a microwave resonance sensor. The apparatus consists of a special resonator unit in the form of an open-bottom cylinder, which is connected to a microwave swept frequency source via a decoupler and a matching unit installed in a waveguide that connects the resonator unit with the microwave source. The microwave generator is fed from a power supply unit through a frequency modulator that may sweep the frequency of microwaves generated by the microwave generator. All the controls can be observed with the use of a display, such as, e.g., a monitor of a personal computer, which may be connected to the microwaveguide line, e.g., via a directed branched waveguide line for directing waves reflected from the resonator, via a reflected wave detector, an amplifier, a synchronous detector, an A/D converter, and a digital voltmeter. A feedback line is going from a direct wave detector, which is installed in a line branched from the microwaveguide between the decoupler and the matching unit, to the power supply unit. The operation resonance frequency of the resonator sensor unit should be somewhere within the range of swept frequencies of the microwave generator.
In operation, the microwave generator generates electromagnetic waves in a certain sweeping range that induces in the resonator sensor unit oscillations on the resonance frequency with a Q-factor on the order of 104 or higher. A distinguishing feature of the resonator of the aforementioned invention is that the design parameters of the resonator unit allow to achieve the aforementioned high Q-factor without physical contact of the sensor unit with the film to be tested. As the surface of the film to be measured constitutes the inner surface of the resonator unit, even a slightest deviation in conductivity will exert a significant influence on the Q-factor. The Q-factor, in turn, defines the height of the resonance peak. As the conductivity directly related to the film thickness, it is understood that measurement of the film thickness is reduced to measurement of the resonance peak amplitudes. This means that super-high accuracy inherent in measurement of the resonance peaks is directly applicable to the measurement of the film thickness or film thickness deviations.
However, since this resonator is a three-dimensional or special device, the measurement surface may have the minimum value on the order of several square millimeters. In such a construction the diameter of the probe practically cannot be reduced beyond the limit of about 1 mm2.
The problems inherent in the method and device of U.S. patent application Ser. No. 386,648 are solved in another patent application Ser. No. 434,625 filed by the same applicants on May 12, 2003.
The system and method of the last-mentioned patent application are aimed at stabilization of the distance between the sensor coil and the surface of the film being measured by constantly measuring the angle of inclination α of a tangent to the curve that represent dependence of the resonance power of the sensor-film system from the distance between the sensor and the film. The aforementioned angle is calculated plotting the resonance curve of a signal, calculating the area between the resonance curve and the abscissa axis, plotting the curve that represent dependence of the aforementioned area from the distance between the sensor coil and the film, measuring the angle α in a preselected point on the last-mentioned curve, and maintaining the distance between the sensor-coil and the film constant by keeping angle α constant in a any measurement point. Angle α can be selected within the range of 0 to 90° C.
The main distinguishing feature of the method and system of the last-mentioned invention is that the measurements are stabilized without the use of complicated measurement devices for distance control but directly via feedback from each measured resonance signal of the coil-film system, i.e., without the use of an additional distance-control sensor. Furthermore, the invention is based on a procedure, wherein the combined resonance signal curve of the sensor-film system is subjected to a specific analysis, the results of which are used for stabilization of the measurement procedure, as well as for measuring the film parameter.
Although the system and the method of patent application Ser. No. 434,625 made a breakthrough in the measurement of thin films and made the resonance sensor technology (RST) applicable to measurement of wide spectrum of thin film characteristics, their potentials are not yet sufficiently applicable for measuring characteristics of non-conductive materials, such as semiconductors or dielectrics.
Furthermore, none of the methods and systems described above was suitable for optimized measurements of thickness in thin dielectric films and electrophysical surface characteristics of semiconductors.